In rotary turbomachines having a compressor stage operating in the transonic region, the problem has been recognized that an excessive Mach number of the fluid flow arises on the suction side of the compressor blades, especially in the vicinity of the hub and/or the housing, due to the finite thickness of the compressor blades, and the consequent effect on the flow conditions. As a result, the danger of boundary layer separation arises on the suction sides of the blades.
In the general field of aircraft construction, and particularly aircraft fuselage construction, it has been known since about 1940 to constrict or narrow the aircraft fuselage so as to have a concave waist in the area of the wing roots in order to provide additional space for the airflow that has been accelerated due to the finite thickness of the wings, especially along the wing roots. In this manner, it has been attempted to avoid the occurrence of excessive velocities and boundary layer separations of the airflow over the wing. Such arrangements are generally known in the context of the so-called surface area rule, and are generally represented in FIG. 1.
Problems would arise if one would attempt to apply the concepts of the above mentioned surface area rule to the construction of the compressor stage of a turbine engine or other rotary turbomachine. If the hub or the housing at the respective root or tip end of the compressor blades of a compressor operating in the transonic region were simply concavely shaped so as to increase the surface area according to the surface area rule, then the resulting configuration would effectively create a supersonic nozzle at any areas operating under supersonic conditions. Such a supersonic nozzle would consequently lead to an undesirable increase of the Mach number of the airflow at these locations. Namely, while such an increase in surface area leads to an improved flow condition under subsonic conditions, this is not true under supersonic conditions, in which the surface area increase would actually cause higher Mach numbers due to the nozzle effect and would thus result in higher shock losses in the locally occurring supersonic fields on the suction sides of the compressor blades in the vicinity of the hub.